1. Field of the Invention
The present invention relates to a surface emitting laser element that includes a cylindrical mesa post in which a plurality of semiconductor layers including an active layer is grown and that emits a laser light in a direction perpendicular to a substrate surface, a surface emitting laser element array, and a method of fabricating a surface emitting laser element.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL, hereinafter, “a surface emitting laser element”) emits a laser light by resonating a light in a direction perpendicular to a growth surface of a plurality of semiconductor layers including an active layer on a substrate. Because the surface emitting laser element is different from an edge emitting laser element in that a cleaving is unnecessary to provide mirrors to form a resonator, a large number of elements can be easily arrayed on a single substrate one-dimensionally or two-dimensionally. The surface emitting laser element has more advantages over the edge emitting laser element in that, for example, a volume of the active layer is extremely small, and laser oscillation can be achieved with an extremely low threshold current and low power consumption. For this reason, the surface emitting laser element draws attentions as a light source for optical communications including optical interconnections, or other various application devices.
Generally, the surface emitting laser element includes a mesa post obtained by etching the semiconductor layers including the active layer on the substrate into a cylindrical shape, and a laser light is emitted from an aperture formed on the mesa post. Generally, a side surface of the mesa post is covered with a dielectric protection film to improve environmental resistance of the surface emitting laser element.
Furthermore, in the surface emitting laser element, a distributed Bragg reflector (DBR) is generally used as the mirrors forming the resonator. Particularly, a dielectric DBR formed of a dielectric multilayer film is known as one that reduces a light absorption loss with a high output of the laser light. A surface emitting laser element including a resonator using the dielectric DBR on a mesa post has been disclosed (see, for example, Japanese Patent Application Laid-open No. 2004-103754).
Because the mesa post of the surface emitting laser element generally has a small diameter of several tens μm, it is difficult to precisely control formation of the dielectric multilayer film as a thick dielectric DBR exclusively on the mesa post area. Therefore, from a viewpoint of productivity, it is easier to form the dielectric DBR on a wide area including the mesa post. With this method, productivity of the surface emitting laser elements each of which includes a mesa post increases. Furthermore, the dielectric multilayer film provided to the side surface of the mesa post can serve as a protection film.
However, when the dielectric multilayer film is formed on the wide area of the surface emitting laser element including the side surface of the mesa post as explained above, a stress caused due to a difference of thermal expansion between the semiconductor layers and the dielectric multilayer film may cause a strain in the active layer in the mesa post. The strain may cause dislocation in the active layer, which leads to a degradation of longterm reliability of the surface emitting laser element.